DESCRIPTION: The proposed research will examine the mechanisms by which the axonal microtubule array is elaborated during axon extension. The ability of neurons to extend axons over long distances is dependent upon the elaboration of highly organized arrays of microtubules. In the proposed experiments, the role of microtublule transport during axon growth will be addressed in living cells. There are 3 specific aims: (1) In the first aim, microtubule behaviors at the neuronal centrosome will be observed within living neurons to determine whether centrosomal microtubules actually detach and move into the axon. (2) In the second aim, fluorescent tubulin will be introduced into neurons in the form of stable microtubule fragments, assembly-incompetent tubulin subunits, or potential intermediate tubulin aggregates, and it will be determined whether tubulin in any of these forms is actively transported down the axon. (3) In the third aim, microtubule behaviors in the axon that underlie the formation of collateral branches will be investigated. Direct observation as well as pharmacologic tools will be used to determine whether microtubules arise within newly-forming branches via transport from the parent axon. Collectively, these efforts will resolve the contribution of microtubule transport to critical features of axon growth.